.\" $OpenBSD: BIO_push.3,v 1.14 2022/12/16 16:02:17 schwarze Exp $
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.\" OpenSSL doc/man3/BIO_push.pod 791bfd91 Nov 19 20:38:27 2021 +0100
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.Dd $Mdocdate: December 16 2022 $
.Dt BIO_PUSH 3
.Os
.Sh NAME
.Nm BIO_push ,
.Nm BIO_pop ,
.Nm BIO_set_next
.Nd manipulate BIO chains
.Sh SYNOPSIS
.In openssl/bio.h
.Ft BIO *
.Fo BIO_push
.Fa "BIO *b"
.Fa "BIO *new_tail"
.Fc
.Ft BIO *
.Fo BIO_pop
.Fa "BIO *b"
.Fc
.Ft void
.Fo BIO_set_next
.Fa "BIO *b"
.Fa "BIO *new_tail"
.Fc
.Sh DESCRIPTION
BIOs can be joined together to form chains.
A chain normally consists of one or more filter BIOs
and one source/sink BIO at the end.
Data read from or written to the first BIO traverses the chain
to the end.
.Pp
Every BIO is a member of exactly one chain.
It is either at the beginning of its chain
or there is exactly one preceding BIO.
It is either at the end of its chain
or there is exactly one following BIO.
If there is neither a preceding nor a following BIO,
it can be regarded as a chain with one member.
Every chain has exactly one beginning and exactly one end.
.Pp
.Fn BIO_push
appends the chain starting at
.Fa new_tail
to the end of the chain that contains
.Fa b .
Unless
.Fa b
is
.Dv NULL ,
it then calls
.Xr BIO_ctrl 3
on
.Fa b
with an argument of
.Dv BIO_CTRL_PUSH .
If
.Fa b
or
.Fa new_tail
is
.Dv NULL ,
nothing is appended.
.Pp
In LibreSSL, if
.Fa new_tail
is not at the beginning of its chain,
the head of that chain up to but not including
.Fa new_tail
is cut off and becomes a separate chain.
For portability, it is best to make sure that
.Fa new_tail
is at the beginning of its chain before calling
.Fn BIO_push .
.Pp
.Fn BIO_pop
removes the BIO
.Fa b
from its chain.
Despite the word
.Dq pop
in the function name,
.Fa b
can be at the beginning, in the middle, or at the end of its chain.
Before removal,
.Xr BIO_ctrl 3
is called on
.Fa b
with an argument of
.Dv BIO_CTRL_POP .
The removed BIO
.Fa b
becomes the only member of its own chain and can thus be freed
or attached to a different chain.
If
.Fa b
is
.Dv NULL ,
no action occurs.
.Pp
.Fn BIO_set_next
appends the chain starting with
.Fa new_tail
to the chain ending with
.Fa b .
.Pp
In LibreSSL, if
.Fa new_tail
is not at the beginning of its chain,
the head of that chain up to but not including
.Fa new_tail
is cut off and becomes a separate chain,
and if
.Fa b
is not at the end of its chain,
the tail of that chain starting after
.Fa b
is cut off and becomes a separate chain.
.Pp
For portability, it is best to make sure that
.Fa b
is at the end of its chain and that
.Fa new_tail
is at the beginning of its chain before calling
.Fn BIO_set_next
and to avoid calling
.Fn BIO_pop
on
.Fa new_tail
afterwards.
.Pp
In LibreSSL, the only built-in BIO type for which
.Xr BIO_ctrl 3
calls with an argument of
.Dv BIO_CTRL_PUSH
or
.Dv BIO_CTRL_POP
have any effect is
.Xr BIO_f_ssl 3 .
.Sh RETURN VALUES
.Fn BIO_push
returns
.Fa b
if it is not
.Dv NULL
or
.Fa new_tail
if it is.
.Pp
.Fn BIO_pop
returns the BIO that followed
.Fa b
in its chain, or
.Dv NULL
if
.Fa b
is
.Dv NULL
or was at the end of its chain.
.Sh EXAMPLES
For these examples suppose
.Sy md1
and
.Sy md2
are digest BIOs,
.Sy b64
is a Base64 BIO and
.Sy f
is a file BIO (see
.Xr BIO_f_md 3 ,
.Xr BIO_f_base64 3 ,
and
.Xr BIO_s_file 3 ,
respectively).
.Pp
If the call
.Pp
.Dl BIO_push(b64, f);
.Pp
is made then the new chain will be
.Sy b64-f .
After making the calls
.Bd -literal -offset indent
BIO_push(md2, b64);
BIO_push(md1, md2);
.Ed
.Pp
the new chain is
.Sy md1-md2-b64-f .
Data written to
.Sy md1
will be digested
by
.Sy md1
and
.Sy md2 ,
Base64-encoded and written to
.Sy f .
.Pp
It should be noted that reading causes data to pass
in the reverse direction.
That is, data is read from
.Sy f ,
Base64-decoded and digested by
.Sy md1
and
.Sy md2 .
If this call is made:
.Pp
.Dl BIO_pop(md2);
.Pp
The call will return
.Sy b64
and the new chain will be
.Sy md1-b64-f ;
data can be written to
.Sy md1
as before.
.Sh SEE ALSO
.Xr BIO_find_type 3 ,
.Xr BIO_new 3 ,
.Xr BIO_read 3
.Sh HISTORY
.Fn BIO_push
first appeared in SSLeay 0.6.0.
.Fn BIO_pop
first appeared in SSLeay 0.6.4.
Both functions have been available since
.Ox 2.4 .
.Pp
.Fn BIO_set_next
first appeared in OpenSSL 1.1.0
and has been available since
.Ox 7.1 .
.Sh CAVEATS
Creating a cyclic chain results in undefined behavior.
For example, infinite recursion or infinite loops may ensue.
.Pp
If it is unknown whether
.Fa b
and
.Fa new_tail
are already members of the same chain and whether joining them would
create a cycle, the calling code can use the following safe idiom:
.Bd -literal -offset indent
BIO *btest;

for (btest = new_tail; btest != NULL; btest = BIO_next(btest))
	if (btest == b)
		/* Bail out because this would create a cycle. */
BIO_push(b, new_tail);  /* This is now safe. */
.Ed
.Pp
The same idiom can be used with
.Fn BIO_set_next
instead of
.Fn BIO_push .
.Pp
Often, the safe idiom is not needed because it is already known that
.Fa b
and
.Fa new_tail
are not members of the same chain, for example when
.Fa b
or
.Fa new_tail
was created right before.
